Introduction
Since the crystalline carbene 1,3-di-l-adamantyl-imidazol-2-ylidene was firstly isolated and characterized by Arduengo et al.,1,2extensive researches emerge involving synthesis of more stable N-heterocyclic carbenes (NHCs) and applications of NHC as ligand of metal-based catalyst or organocatalyst.3Particularly, the success in catalytic benzoin condensations promotes the NHC catalysis as an important alternative in constructing new C-C bonds and C-X (X = N, O, S) bonds.4-9 Inspired by the ‘umpolung’ principle used in this reaction,10,11the NHC-catalyzed Stetter reaction,12 benzoin reaction,13 homoenolate reaction, and especially cycloaddition/annulation reactions14-18 have contributed a number of novel approaches to constructing various heterocyclic structures. Inversion of the conventional reactivity of substrates, typically such as aldehyde, ketone, ketene, and so on, is the basic underline principle behind these reactions (Scheme 1).19-21 More interestingly, catalysis using this concept has been extended to the umpolung of Michael acceptors22-25 and aldimines.26-33Detailed theoretical studies reported by our34-42 and other groups43-51 provide perspective at the molecular level to get insights into the catalytic mechanism and chemo-, regio-, and/or stereoselectivities.52,53